mmaprototype: allow rebalancing between stores on the same node

Previously, when MMA constructed the candidate set, it excluded stores on nodes
that already had replicas, including the current node itself. However,
rebalancing replicas across different stores on the same node should be allowed.
This commit corrects that logic. Note that the store itself has already been
excluded earlier (see https://github.com/cockroachdb/cockroach/blob/da4a31068dc9f268d423aa4d5fd72b29c61942bc/pkg/kv/kvserver/allocator/mmaprototype/allocator_state.go#L614-L623).

Author: wenyihu6

pkg/kv/kvserver/allocator/mmaprototype/allocator_state.go

@@ -673,6 +673,12 @@ func (a *allocatorState) rebalanceStores(
 			// Also exclude all stores on nodes that have existing replicas.
 			for _, replica := range rstate.replicas {
 				storeID := replica.StoreID
+				if storeID == store.StoreID {
+					// We don't exclude other stores on this node, since we are allowed to
+					// transfer the range to them. If the node is overloaded or not fdOK,
+					// we have already excluded those stores above.
+					continue
+				}
 				nodeID := a.cs.stores[storeID].NodeID
 				for _, storeID := range a.cs.nodes[nodeID].stores {
 					storesToExcludeForRange.insert(storeID)